Spin torque oscillator having multiple fixed ferromagnetic layers or multiple free ferromagnetic layers

ABSTRACT

A spin torque oscillator and a method of making same. The spin torque oscillator is configured to generate microwave electrical oscillations without the use of a magnetic field external thereto, the spin torque oscillator having one of a plurality of input nanopillars and a nanopillar having a plurality of free FM layers.

This application is a divisional of U.S. patent application Ser. No.12/973,269, filed Dec. 20, 2010, the content of which is herebyincorporated by reference.

FIELD

Embodiments of the invention relate to spin torque oscillators.

BACKGROUND

Spin torque oscillators have been first demonstrated in the year 2003.See S. I. Kiselev et al., Nature 425, 380 (2003). Since then, they havebeen shown to provide a reliable option as microwave signal generators.They typically provide a smaller size than other microwave oscillators,and are defined mainly by the size of their ferromagnetic nanopillars,which typically have a size below about 200 nm. The frequency of a spintorque oscillator may be tuned by varying the current passed through thesame. In general, the operation of spin torque oscillators relies onprecession of magnetization in the free (or “active”) ferromagnetic (FM)layer under the action of spin torque due to electrons crossing thenon-magnetic layer (such as copper or MgO, for example) from the FMlayer. The variation or precession of magnetization is then converted toan electric signal via the effect of magnetoresistance, which refers tothe change in resistance of the stack of materials of the spin torqueoscillator based on the relative orientations of magnetization in thefree and fixed FM layers.

Currently, microwave oscillators based on nanomagnets and theirprecession by the spin torque effect require an external magnetic fieldfor their operation. Under such circumstances, one would need either apermanent magnet or a wire with current in order to create an externalmagnetic field. In the case of a permanent magnet, however,disadvantageously, the spin torque oscillator would take additionalspace, and further, interference would be created with other parts ofthe circuit. In the case of a wire, the arrangement would lead toconstantly dissipated Joule heat, which could be disadvantageous to thecircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention may be better understood by referringto the following description and accompanying drawings that are used toillustrate embodiments of the invention. In the drawings:

FIG. 1 a is a perspective view of a spin torque oscillator according tothe prior art;

FIG. 1 b is a top plan view of the spin torque oscillator of FIG. 1 a;

FIG. 2 is a plot of magnetization energy versus angle of magnetizationin plane of a free FM layer in a spin torque oscillator of the priorart;

FIG. 3 a is a top plan view of a spin torque oscillator according to afirst embodiment;

FIG. 3 b is a cross section view of FIG. 3 a along lines B-B;

FIG. 4 depicts successive snapshots of simulated magnetizations in topplan view within a free FM layer according to the first embodiment;

FIG. 5 a shows a perspective view of a spin torque oscillator accordingto a second embodiment;

FIG. 5 b shows a top plan view of the spin torque oscillator of FIG. 5a;

FIG. 5 c shows magnetization schemes on two free ferromagnetic layersaccording to the second embodiment;

FIG. 6 is a flowchart showing a method according to embodiments; and

FIG. 7 is a system incorporating a spin torque oscillator according toembodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure an understanding of this description.

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the claimed subject matter may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the subject matter. It is to be understood thatthe various embodiments, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein, in connection with one embodiment, maybe implemented within other embodiments without departing from thespirit and scope of the claimed subject matter. In addition, it is to beunderstood that the location or arrangement of individual elementswithin each disclosed embodiment may be modified without departing fromthe spirit and scope of the claimed subject matter. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the subject matter is defined only by the appendedclaims, appropriately interpreted, along with the full range ofequivalents to which the appended claims are entitled. In the drawings,like numerals refer to the same or similar elements or functionalitythroughout the several views, and that elements depicted therein are notnecessarily to scale with one another, rather individual elements may beenlarged or reduced in order to more easily comprehend the elements inthe context of the present description.

Embodiments are predicated on the well-known principle that aspin-polarized electrical current can apply a torque to a ferromagnetthrough direct transfer of spin angular momentum. Thus, a magneticmultilayer structure may convert energy for example from a directelectrical current into high-frequency magnetic rotations which may beapplied in devices such as microwave oscillators. An example of aconventional device that achieves the above result is shown in FIGS. 1 aand 1 b, along with a supporting plot at FIG. 2.

Referring in particular to FIGS. 1 a and 1 b, the nanopillar 110 of aconventional spin torque oscillator 100 is shown in side view and in topplan view, respectively. Nanopillar 110 includes a material stack 112comprising a free ferromagnetic (FM) layer 114 separated from a fixed FMlayer 116 by a non-magnetic layer 118. The FM layers may include any FMmaterial, such as, for example, cobalt, a cobalt-iron alloy, ornickel-iron alloy, a cobalt-nickel alloy, or iron-platinum alloy. The FMlayer 116 can be a multi-layer combination of several alloys. Thenon-magnetic layer 118 may for example include a non-magnetic metal suchas copper; in this case the layers 114, 118 and 116 together are termeda “spin valve”. Alternatively, the non-magnetic layer 118 may forexample include a dielectric/non-magnetic material, such as aluminumoxide (Al₂O₃) or magnesium oxide (MgO); in this case the layers 114, 118and 116 together are a “magnetic tunnel junction”. A pinninganti-ferromagnetic layer (AFM layer) 120 is disposed such that the fixedFM layer is sandwiched between it and the non-magnetic layer. The roleof the pinning AFM layer is to prevent the fixed FM layers fromundergoing rotation as a result of spin transfer torque. The AFM layermay comprise any exchange material such as, for example, iron-manganesealloy or a platinum-manganese alloy Material stack is in turn sandwichedbetween a top electrode 122 and a bottom electrode 124 as shown. Theelectrodes may be made of any non-magnetic conductive material forexample, such as copper or gold. For example, the free FM layer mayinclude a 3 nm layer of Co, the non-magnetic layer may include a 3 nmlayer of Cu, the fixed FM layer may include a 40 nm layer of Co, thepinning AFM layer may include a 20 nm layer of PtMn and the top andbottom electrode may include layers of Cu. In its shown elliptical crosssection, the nanopillar 110 may have a long axis measuring about 130 nmand a short axis measuring about 70 nm. Transmission of electrons by wayof direct current by virtue of a voltage applied across material stack112 drives the electrons through the fixed FM layer 116 toward thenon-magnetic layer, applying a torque to the free FM layer 114. Anoscillation in the magnetization of the free FM layer 114 relative tothat of the fixed FM layer 116 may result, changing the resistance ofthe nanopillar 110. Under direct current conditions, such magneticdynamics produce a time-varying voltage, which in turn may generate asignal having a frequency in the microwave range. As best seen in FIG. 1b, the nanopillar is elliptical in cross section. Under such aconfiguration, the oscillations tend to be symmetric relative to thedirection of the magnetization moment in the free FM layer 114. In sucha case, voltage signals would occur only under two stable states of thefree FM layer 114 as will be explained with respect to FIG. 2 below.

Referring next then to FIG. 2, a schematic plot of energy of ananomagnet such as nanopillar 110 of FIGS. 1 a and 1 b is shown versusthe angle of magnetization in plane of a free FM layer comparable tofree FM layer 114. Two stable magnetization states exist formagnetization along the long axis of the ellipse marked “1” on the plot.An external magnetic field H is applied, the energy of one state becomeslower than that of the other as seen on the right hand side of FIG. 2.At a larger magnetic field, only one stable state may remain. Thus,referring back to FIG. 1, to ensure proper signal strength, an externalmagnetic field must be applied to the nanopillar 110 as shown by arrow Hin order to leave a single state around which the magnetization in thefree FM layer 114 can rotate.

Embodiments do away with the necessity of providing an external magneticfield H in order to ensure proper microwave signal strength from a spintorque oscillator. By a magnetic field that is “external” to a spintorque oscillator, what is meant is a magnetic field which is notgenerated by virtue of running a current through the spin torqueoscillator. Thus, embodiments provide a spin torque oscillatorcomprising a nanopillar configured to generate microwave electricaloscillations without the use of a magnetic field external thereto.According to a first embodiment, the spin torque oscillator includes aplurality of nanopillars including an output nanopillar and at least twoinput nanopillars, wherein the nanopillars are insulated from oneanother and include respective fixed magnetic layers. The firstembodiment allows independent currents to be injected through the inputnanopillars, their values and/or polarities chosen to create anoscillating magnetic vortex by virtue of a spatially varying spin torquein the free FM layer generated by the independently injected inputcurrents. One nanopillar may then be used for the output signal.According to a second embodiment, the spin torque oscillator has asubstantially circular cross-section, is free of any pinning layer, andcomprises a first free ferromagnetic layer and a tunnel junctiondisposed on the first free ferromagnetic layer, and a second freeferromagnetic layer different in at least one of thickness andsaturation magnetization from the first free ferromagnetic layer, thesecond free ferromagnetic layer being disposed on the tunnel junction.Here again, an external magnetic field is not needed. One forms two freeFM layers that lack the shape anisotropy provided by a free FM layerhaving an elliptical cross section. By virtue of the near circular crosssections thereof, the two free FM layers of the second embodiment lackpreferred/lower energy states. Thus, the spatially varying magnetizationis generated within the free FM layer without the need for an externallyapplied magnetic field.

The first and second embodiments will now be described in further detailbelow with respect to FIGS. 3 a-4 and to FIGS. 5 a-5 c, respectively.

Referring first to FIGS. 3 a-4, an example is provided of a firstembodiment. As seen in FIG. 3 a, where a spin torque oscillator 300according to a first embodiment is shown in top plan view. The exemplaryspin torque oscillator 300 may include three input nanopillars 310, 310′and 310″, and an output nanopillar 311 as shown, all disposed on acommon base 315, although the first embodiment as described above mayinclude two input nanopillars or more. Base 315, as may be best seenwith reference to FIG. 3 b, includes a non-magnetic layer 318 and a freeFM layer 314 coextensive therewith. As seen in FIG. 3 a, base 315 on theone hand, and input nanopillars 310, 310′ and 310″ on the other hand,may preferably have elliptical cross sections/elliptical top plan views.Elliptically shaped cross sections/top plan views for the nanopillars310, 310′, 310″, and 311 ensure that the fixed FM layers 316 in themhave the stable magnetization state along the long axis of the ellipse.

Referring next to FIG. 3 b, a cross-sectional view is shown of the spintorque oscillator 300 of FIG. 3 a along broken line B-B. As seen in FIG.3 b, the plurality of nanopillars 310, 310′, 310″ and 311 each include amaterial stack 312 comprising a fixed FM layer 316 and a pinning AFMlayer 320 disposed on the fixed FM layer 316. The AFM layer maycomprise, for example, a platinum-manganese alloy. The spin torqueoscillator 300 further includes a non-magnetic layer 318 which mayextend under all of the nanopillars 310, 310′, 310″ and 311, and a freeFM layer 314 coextensive with the non-magnetic layer 118. The FM layersmay include any FM material, such as, for example, cobalt, a cobalt-ironalloy, a nickel-iron alloy or a iron-platinum alloy. The non-magneticlayer 318 may for example include any dielectric/non-magnetic material,such as for example copper. According to one embodiment, thenon-magnetic layer 318 may include a dielectric layer, such as, forexample, MgO. Material stack 312 along with non-magnetic layer 318 andfree FM layer 314 are sandwiched between a ground electrode 322 andrespective top electrodes 324 above each material stack 312, theelectrodes comprising for example non-magnetic metals such as copper,gold or platinum. The electrodes may be metal layers in an integratedcircuit, for example a microprocessor, a microwave transceiver orsensor. In the shown embodiment, the magnetization of the free layer 314is variable and in general spatially non-uniform by virtue of thepositioning of the input nanopillars and the possibility of providingindependent respective input currents thereto. Magnetization in thefixed layers 316 is kept constant by the adjacent pinning AFM layers320. A transmission of electrons by way of direct independent currentsinto each of nanopillars 310, 310′ and 310″ by virtue of differingrespective voltages to the same drives electrons through each respectivefixed FM layer 316 toward or away from the non-magnetic layer, applyinga torque to the free FM layer 114. The direction of the torque generatedby each input nanopillar 310, 310′ and 310″ (whether clockwise orcounterclockwise) depends on the direction of electron flux within theinput nanopillar. Thus, where a positive voltage is applied to an inputnanopillar, such as input nanopillars 310 and 310′ in FIGS. 3 a and 3 b,the direction of electron flux within that input nanopillar will be awayfrom the free FM layer, and the resultant magnetic torque within thefree FM layer will be pointing downward, as shown by arrows Td in FIG. 3a. On the other hand, where a negative voltage is applied to an inputnanopillar, such as input nanopillar 310″ in FIGS. 3 a and 3 b, thedirection of electron flux within that input nanopillar will be towardthe free FM layer 314, and the resultant magnetic torque within the freeFM layer will be pointing upward, as shown by arrow Tu in FIG. 3 b.Thus, for each respective input nanopillar 310, 310′ and 310″, arespective component to an oscillation in the magnetization of the freeFM layer 314 relative to that of the associated fixed FM layer 316 mayresult, each of those components being a function of the positioning ofthe input nanopillar and of the amount and/or polarity of currentsupplied thereto. Because of the differing input currents supplied toeach of input nanopillar, the torque resulting from each within the freeFM layer may be different, and hence an asymmetry in the precessionwithin the free FM layer may be generated which results in a magneticoscillation of the same. Such magnetic dynamics produce a time-varyingvoltage, which in turn may generate a signal having a frequency in themicrowave range.

In the first embodiment of the invention, a positioning of respectivenanopillars with respect to one another may be determined as a functionof the desired pattern of magnetization change within the free FM layer.As suggested above, a positioning of input nanopillars may be determinedby virtue of the torque direction that they may be desired to producedwithin the free FM layer. In general, the output nanopillar maypreferably be disposed at a location corresponding to a largestmagnetization change within the free FM layer. For example, if themagnetization within the free FM layer is to move in a vortex pattern,it may be preferable to place the output nanopillar at the center of theshape defined by the top plan view of the free FM layer as the centerposition corresponds to a location corresponding to a largestmagnetization change within the free FM layer.

According to one embodiment, as suggested for example in FIG. 3 a, theinput nanopillars 310, 310′ and 310″ may be disposed to surround theoutput nanopillar 311. Coordinates of the centers of the pillars, aswell as dimensions of the base layer 315, are indicated in FIG. 3 a interms of the minimum width of the pillar L, and minimum gap between thepillars G. Thus, in the shown example, base layer 315 may have a majorradius a and minor radius given by equations (1) and (2) below:a=1.5b  Eq. (1)b=2G+1.5L  Eq. (2)In addition, nanopillars 310, 310′ and 310″ in the shown example have xcoordinates of their centers given by the following equations (3), (4)and (5) below:x(310)=0.75(L+G)  Eq. (3)x(310′)=0.75(L+G)  Eq. (4)x(310″)=−1.5L  Eq. (5)

Nanopillars 310, 310′ and 310″ in the shown example have y coordinatesof their centers given by the following equation (6), (7) and (8) below:y(310)=0.87(L+G)  Eq. (6)y(310′)=−0.87(L+G)  Eq. (7)y(310″)=0  Eq. (8)

Also shown in FIG. 3 a the spin torques Td and Tu in the free FM layerdisposed under the nanopillars 310, 310′ and 310″ indicate the overalldirection of rotation that promotes creation of a magnetic vortex.According to the first embodiment, the proposed arrangement of the inputnanopillars surrounding the output nanopillar, and the polarity of thecurrent passed through the input nanopillars (two positive and onenegative direction of the current) will combine into a rotating patternof magnetization change, and will promote the formation of the shownvortex.

FIG. 4 shows successive snapshots from left to right and from top row tobottom row of simulated magnetization directions in top plan view withina free FM layer such as free FM layer 314 of FIGS. 3 a and 3 b takenwithin intervals of 0.1 ns. The images indicate that the position of thevortex V rotates around the center of the nanomagnet, which, for theshown simulation, was chosen to be an elliptic cylinder 210 nm×140 nm×3nm with a current of 4 mA in each nanopillar, the three nanopillars havepolarities of applied voltages and hence the polarities of the currentsas indicated in FIG. 3 b. In an example, voltage of magnitude of 4 voltsis applied to each pillar. The simulations show that the center of avortex will move in an elliptic orbit around the center of the ellipse.Thus the direction of magnetization under the output pillar will rotateand the resistance of the material stack will oscillate accordingly.Application of voltage to the output pillar will result in anoscillating output current with a frequency of the order of 1-10 GHz.Voltage of, for example 0.5 volts, is applied to the output pillar.

Referring next to FIGS. 5 a-5 c, an example of a second embodiment isshown. As seen in FIGS. 5 a and 5 b, a nanopillar 510 of a spin torqueoscillator 500 is shown in side view and in top plan view, respectively.Nanopillar 510 includes a material stack 112 comprising a first freeferromagnetic (FM) layer 514 separated from a second free FM layer 517by a non-magnetic layer 518 layer. According to the second embodiment,the two free FM layers have differing thicknesses and/or differingmagnetization saturation values. Saturation magnetization value is thelimit of magnetization value achieved for magnetized ferromagnet at lowtemperature. The FM layers may include any FM material, such as, forexample, cobalt, a cobalt-iron alloy, a nickel-iron alloy or airon-platinum-iron alloy. The non-magnetic layer 518 may for exampleinclude any dielectric/non-magnetic material, such as copper or MgO.Material stack is in turn sandwiched between a top electrode 522 and abottom electrode 524 as shown. The electrodes may be made of anynon-magnetic conductive material for example, copper or gold, and maycorrespond to metal lines in an integrated circuit. In the secondembodiment, no pinning AFM layers are present, and therefore no fixed FMlayers exist. In addition, by virtue of a near circular cross section ofthe nanopillar 510, no stable configurations exist. A transmission ofelectrons by way of direct current by virtue of a voltage applied acrossmaterial stack 512 drives the electrons through the first free FM layer514 and second free FM layer 517, applying a torque to each of the freeFM layers. Both of the free FM layers precess, albeit with differentangular velocities by virtue of their differing thicknesses and/ordiffering saturation magnetization values.

Referring next to FIG. 5 b, the nanopillar is nearly circular in crosssection. Under such a configuration, there are no lower energyconfigurations/stable magnetic states, by virtue of the lack of shapeanisotropy as would exist in an elliptical cross section, for example.Under direct current conditions, by virtue of the changing of therelative angle of the two magnetizations generated in the respectivefree FM layers 514 and 517, microwave electrical oscillations may begenerated in the voltage across nanopillar 510, since the resistance ofthe material stack 512 will vary with the relative angle ofmagnetizations.

A simulation regarding the second embodiment as shown by way of examplein FIGS. 5 a and 5 b has been performed. The Landau-Lifshitz-Gilbertequations for the evolution of magnetization in two free FM layers(written here in the approximation of a uniform magnetization in eachlayer) are:

$\begin{matrix}{\frac{\mathbb{d}M_{1}}{\mathbb{d}t} = {{- {\gamma\left\lbrack {M_{1} \times B} \right\rbrack}} + {\frac{\alpha}{M_{1}}\left\lbrack {M_{1} \times \frac{\mathbb{d}M_{1}}{\mathbb{d}t}} \right\rbrack} - {\frac{\gamma\;\hslash\; P_{2}J}{2\; M_{s\; 1}^{2}M_{s\; 2}{{et}_{1} \cdot {g(\theta)}}}\left\lbrack {M_{1} \times \left\lbrack {M_{1} \times M_{2}} \right\rbrack} \right\rbrack}}} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$Where B is magnetic field equal to zero for this example, M_(1,2) is themagnetization in each layer, M_(s1,2) is the saturation magnetization ineach layer, t_(1,2) is the thickness of each layer, Θ=Θ₁−Θ₂ is thedifference of angles of magnetizations for each layer, J is the currentdensity, P_(1,2) is the spin polarization of carriers for each layer.Therefore if both layers are free, the difference of the magnetizationangles evolves as:

$\begin{matrix}{\frac{\mathbb{d}\theta}{\mathbb{d}t} = {\frac{\gamma\;\hslash\; J\;\sin\;\theta}{2\;{{eg}(\theta)}}\left( {\frac{P_{2}}{t_{1}M_{1}} - \frac{P_{1}}{t_{2}M_{2}}} \right)}} & {{Eq}.\mspace{14mu}(10)}\end{matrix}$It is this difference that determines the resistance of the stack interms of resistances of the parallel and anti-parallel configurations:R=R _(p)(1÷cos θ)/2+R _(sp)(1−cos θ)/2  Eq. (11)We have concluded based on the above that the angle evolves in aquasi-harmonic fashion. Thus the resistance of the stack oscillates. Thefrequency of oscillations is proportional to the current. This enables avery efficient tuning via the current value, unlike the conventionaloscillator explained above. The reason for it is that the conventionalspin torque oscillator relies on eigen-frequencies of the magnetizationmodes in the nanomagnet, while with respect to embodiments theoscillations are driven by the current.

Referring to FIG. 5 c, a scheme of magnetizations is shown acting on thetwo free FM layers denoted M1 and M2 (slightly off-set for clarity),which could correspond for example to free FM layers 514 and 517 of FIG.5 a. As seen in FIG. 5 c, the arrow denoted with Θ1,2's correspond tothe direction of magnetization, and the arrows marked T1,2 correspond tothe torques acting on respective ones of the layers. FIG. 5 c showsamong others that the magnetizations in the respective layers may havedifferent magnitudes and directions, and that the torques generated inrespective layers are a function of the magnetizations.

Referring next to FIG. 6, a method embodiment is depicted in flowchartformat. A method embodiment 600 at block 610 includes making a spintorque oscillator configured to generate microwave electricaloscillations in the absence of two stable oscillation states and withoutthe use of a magnetic field external thereto. Making a spin torqueoscillator according to embodiments may include, as shown at block 620providing a spin torque oscillator having a plurality of inputnanopillars or it may include, as shown at block 630, providing a spintorque oscillator having a nanopillar having a plurality of free FMlayers.

Referring to FIG. 7, there is illustrated one of many possible systems900 in which embodiments of the present invention may be used. In oneembodiment, the electronic arrangement 1000 may include an integratedcircuit 710 including a spin torque oscillator, such as oscillator 300or 500 of FIGS. 3 a-3 b or 5 a-5 b. Arrangement 1000 may further includea microprocessor, a transceiver chip, or a sensor chip. In an alternateembodiment, the electronic arrangement 1000 may include an applicationspecific IC (ASIC). Integrated circuits found in chipsets (e.g.,graphics, sound, and control chipsets) may also be packaged inaccordance with embodiments of this invention.

For the embodiment depicted by FIG. 7, the system 900 may also include amain memory 1002, a graphics processor 1004, a mass storage device 1006,and/or an input/output module 1008 coupled to each other by way of a bus1010, as shown. Examples of the memory 1002 include but are not limitedto static random access memory (SRAM) and dynamic random access memory(DRAM). Examples of the mass storage device 1006 include but are notlimited to a hard disk drive, a compact disk drive (CD), a digitalversatile disk drive (DVD), and so forth. Examples of the input/outputmodule 1008 include but are not limited to a keyboard, cursor controlarrangements, a display, a network interface, and so forth. Examples ofthe bus 1010 include but are not limited to a peripheral controlinterface (PCI) bus, and Industry Standard Architecture (ISA) bus, andso forth. In various embodiments, the system 900 may be a wirelessmobile phone, a personal digital assistant, a pocket PC, a tablet PC, anotebook PC, a desktop computer, a set-top box, a media-center PC, a DVDplayer, and a server.

Advantageously, according to embodiments, a very compact and low powermicrowave spin torque oscillator may be created that does not requirethe use of an external magnetic field, in this way not only saving spaceand power, but further minimizing interference with other parts of thecircuit and minimizing the dissipation of Joule heat. Such spin torqueoscillators may be used to decrease the power of clocking and oftransmitting wireless devices, thereby extending the battery lifetime ofportable computing devices. According to the first embodiment,advantageously, a spin torque oscillator may be provided that allow atuning of the frequency and direction magnetic oscillations therein byadjusting voltage polarity and amount. With respect to the secondembodiment, additionally, a wider tunability of oscillation frequency isafforded by the multiple free FM layer design. Moreover, doing away witha pinning FM layer advantageously lowers fabrication costs and thechance of failures.

The foregoing description is intended to be illustrative and notlimiting. Variations will occur to those of skill in the art. Thosevariations are intended to be included in the various embodiments of theinvention, which are limited only by the scope of the following claims.

What is claimed is:
 1. A spin torque oscillator configured to generatemicrowave electrical oscillations without the use of a magnetic fieldexternal thereto, the spin torque oscillator having a nanopillar thatincludes a plurality of free ferromagnetic (FM) layers, wherein thenanopillar includes no fixed magnetic layers.
 2. The oscillator of claim1, wherein each of the plurality of free FM layers has a cross-sectionthat is not substantially elliptical.
 3. The oscillator of claim 2,wherein the nanopillar includes no pinning layers.
 4. The oscillator ofclaim 2, wherein the nanopillar comprises: a first free FM layer; atunnel junction on the first free FM layer; and a second free FM layeron the tunnel junction; wherein the second free FM layer is different inat least one of thickness and saturation magnetization from the firstfree FM layer.
 5. The oscillator of claim 4, wherein the nanopillarincludes no pinning layers.
 6. The oscillator of claim 4, wherein thefirst and second free FM layers have no shape anisotropy.
 7. Theoscillator of claim 2, wherein the nanopillar comprises: a first free FMlayer; a tunnel junction on the first free FM layer; and a second freeFM layer on the tunnel junction; wherein the second free FM layer isdifferent in thickness from the first free FM layer.
 8. The oscillatorof claim 7, wherein the second free FM layer is different in saturationmagnetization from the first free FM layer.
 9. The oscillator of claim2, wherein the nanopillar comprises: a first free FM layer; a tunneljunction on the first free FM layer; and a second free FM layer on thetunnel junction; wherein the second free FM layer is different insaturation magnetization from the first free FM layer.
 10. Theoscillator of claim 2, wherein first and second free FM layers, includedin the plurality of free FM layers, have different magnetic magnitudesand directions from each other.
 11. The oscillator of claim 1, whereinfirst and second free FM layers, included in the plurality of free FMlayers, have no shape anisotropy.
 12. A method of making a spin torqueoscillator configured to generate microwave electrical oscillationswithout the use of a magnetic field external thereto, comprisingproviding a spin torque oscillator having a nanopillar that includes aplurality of free ferromagnetic (FM) layers, wherein the nanopillarincludes no fixed magnetic layers.
 13. The method of claim 12, whereineach of the free FM layers has a substantially circular cross-sectionand the nanopillar includes no pinning layers, the method comprising:providing a first free FM layer; providing a tunnel junction on thefirst free FM layer; and providing a second free FM layer on the tunneljunction; wherein the second free FM layer is different in at least oneof thickness and saturation magnetization from the first free FM layer.14. A system comprising an integrated circuit including: a device layer;a plurality of inter-layer dielectric (ILD) layers disposed on thedevice layer; a plurality of metal lines interleaved between the ILDlayers; a spin torque oscillator disposed between two of the pluralityof metal lines such that the metal lines correspond to electrodesthereof, the spin torque oscillator being configured to generatemicrowave electrical oscillations without the use of a magnetic fieldexternal thereto, the spin torque oscillator having a nanopillar thatincludes a plurality of free ferromagnetic (FM) layers; wherein thenanopillar includes no pinning layers and no fixed magnetic layers. 15.The system of claim 14, wherein each of the plurality of free FM layershas a substantially circular cross-section.
 16. The system of claim 15,wherein the nanopillar comprises: a first free FM layer; a tunneljunction on the first free FM layer; and a second free FM layer on thetunnel junction; wherein the second free FM layer is different inthickness from the first free FM layer.
 17. The oscillator of claim 16,wherein the second free FM layer is different in saturationmagnetization from the first free FM layer.
 18. The system of claim 14,wherein the nanopillar comprises: a first free FM layer; a tunneljunction on the first free FM layer; and a second free FM layer on thetunnel junction; wherein the second free FM layer is different insaturation magnetization from the first free FM layer.